1. Field of the Invention
The present invention relates to an analog front end (AFE) circuit and to a method of compensating for DC offset in the analog front end circuit. More particularly, the present invention relates to an analog front end (AFE) circuit and a method of compensating for DC offset in the analog front end circuit in a manner that eliminates the undesired DC offset in a high-precision system.
2. Description of the Related Art
Analog systems and digital systems are commonly implemented in an integrated circuit using system on-chip (SOC) technology. Such systems commonly include an analog front end (AFE) circuit. The analog front end (AFE) circuit operates as an interface between an external input terminal, through which analog signals are input, and a digital signal processing unit that processes the received signals in digital format.
The analog front end (AFE) circuit includes an amplifier and an analog-to-digital converter (ADC). The amplifier amplifies the received analog signals, and the ADC converts the amplified analog signals into digital signals.
The AFE circuit is widely used in various devices, such as down converters for wireless digital communication devices, digital image scanners, digital cameras and voice codecs, and the like. The voice codec device is commonly used in wired or wireless telephone and digital answering machines for transmitting and recording voice data and music data.
Audio devices and video devices generally require high precision and low power consumption. Recently, multi-function voice codec has been developed and is applied to various applications as wired systems and wireless systems become increasingly integrated, and as communication systems and broadcasting systems become increasingly integrated.
In voice applications, the AFE circuit requires high resolution and low bandwidth operation. Thus, the AFE circuit adopts oversampling techniques and noise shaping technology for voice applications. In addition, the AFE circuit commonly provides a programmable gain function and a programmable attenuation function so as to interface with various input/output devices such as microphones and speakers, and the like.
The AFE circuit includes the ADC unit for processing digital signals in the rear stage of the AFE circuit. However, the analog input signals have various amplitudes depending upon the application. When the analog input signal has a small amplitude and the analog input signal is not amplified, the voltage level of an input signal input into the ADC is small, thus the ADC may not provide adequate conversion capabilities. For this reason, an amplifier is disposed in the front end of the ADC.
However, when the amplifier amplifies other, undesired, signals during the amplification operation, the ADC does not provide a desired conversion result, and a large amount of noise is therefore generated in the system having the AFE circuit. For example, in the case where the analog signal has a DC component and an AC component, the AC characteristics of the system, such as SNR (Signal to Noise Ratio), are enhanced only when the AC component is amplified and the DC component is not amplified. When the DC component signal is also amplified, the AFE circuit may operate abnormally, since the operation condition of the rear stage of the AFE circuit may be changed. Harmonic distortion may occur because signals may be clipped when the rear stage of the AFE circuit does not operate within the normal operational range.
Therefore, it is desirous to minimize the DC component of the signal that is amplified by the amplifier of the AFE circuit, in order to minimize the change in the operation condition of the rear stage of the AFE circuit.
FIG. 1 is a schematic view showing a conventional analog front end (AFE) circuit.
Referring to FIG. 1, the AFE circuit 10 includes a subtractor 12, an amplifier 14, an analog-to-digital converter (ADC) 14, a quantizer 18 and an offset compensation circuit 20.
According to the conventional AFE circuit 10, the DC offset of the amplifier 14 is added to the DC offset of the ADC 16, and then the resulting undesired DC offset is compensated. Thus, the undesired DC offset has large value. The conventional AFE circuit requires operation within preferred conditions (or range) and within preferred circuit characteristics in order to compensate for the large value of the undesired DC offset.
In addition, since the DC offset of the amplifier 14 is added to the DC offset of the ADC 16, the combined DC offsets of the amplifier 14 and the ADC 16 are compensated for at the same time. Thus, it is difficult to precisely compensate for the undesired DC offset, and thus, the conventional AFE circuit may not be applicable to use in systems requiring high-precision operation.
In addition, when external noise signals are input to the AFE circuit of FIG. 1, or when the AFE circuit has operational characteristics that are sensitive to the noise signals, the ADC 16 converts the signals having the noise signals into digital data, the digital data is fed back through the quantizer 18 and offset compensation circuit 20 to subtractor 12 and then is amplified by the amplifier 14. Thus, the result of the DC offset compensation may be worsened.